Improve maintenance handling of default group member/admin roles

Issue Summary

Adding a group member or group admin, when a default role is set, automatically adds the default role to that user's group roles. If you then go and change the default role, the previously assigned default group role is not replaced with the new default role for existing users. Furthermore, when an admin user is demoted to a regular user, the old default role (in the og_user_roles table) is not removed. There is no way to remove the old role in the UI, and that user continues to enjoy the privileges of that role.

I'm not sure what the best approach is to fix this. The og_user_roles table makes no distinction between roles assigned automatically by default, and roles assigned manually. If you just remove all og_user_roles matching the former default role, you will remove manually assigned roles of that type. I cannot think of a solution short of adding a field to the og_user_roles table.